Various metering devices of this type have been proposed before and reference may be made to a WO90/10190A and WO/007561 which disclose arrangements of this general type.
The valving arrangements used in the prior proposed metering c have been relatively complex and bulky.
The present invention seeks to provide an improved metering device, of the general type discussed above, in which the valving arrangement is relatively simple and capable of being manufactured as a relatively small unit. Certain embodiments of the invention seek to provide a metering device which incorporates facilities to carry out checks on the operation of the metering device.